Engagement data is time-based, and the event dates might fall outside the default lookback window (90 days). Try increasing the lookback window (released November 2024) under Segment Properties. Note that it cannot exceed 24 months on a standard publish segment. For rapid publish segments, it cannot exceed 7 days.
Also, while your segment population on a standard publish segment can be selected based on the last 2 years of related engagement data, the segment activation only supports including records from a 90-day lookback window. Specific engagement records might not be included in the activation if they fall outside the 90-day lookback window. And that lookback window is reduced to 7 days for activations on rapid publish segments.
See this help document for more detail.